Wireless communications systems use multiple access protocols, such as time-division multiple access (TDMA), frequency-division multiple access (FDMA), code-division multiple access (CDMA), and space-division multiple access (SDMA) protocols, to enable wireless communications between base transceiver stations and multiple subscriber units. Typically, a wireless communications system includes multiple base transceiver stations that are spaced to create subscriber cells. Subscriber units within the subscriber cells, which may include mobile or fixed units, exchange information between a nearby base transceiver station over a dedicated radio frequency.
The use of wireless communications system is rapidly expanding beyond the exchange of voice communications to include the exchange of bandwidth intensive multimedia data transmissions. For example, a single wireless link between a base transceiver station and a subscriber unit may be utilized to simultaneously exchange voice, video, and data. In order to effectively deliver different media types over the same wireless link, wireless links must be able to reliably maintain certain minimum performance levels. Some common performance levels are defined in terms of data rate, capacity, delay, loss rate, and error rate.
The performance of a wireless link is primarily a function of the operating parameters of a transmitter-receiver pair and the channel characteristics that exist between the transmitter-receiver pair. The operating parameters of a transceiver-receiver pair include the transmit power, the coding scheme, the modulation rate, and the guard region. The operating parameters can typically be controlled to a relatively high degree of accuracy. The channel characteristics that exist between the transmitter-receiver pair include ambient noise, co-channel interference, adjacent channel interference, propagation path loss, and multi-path fading. In contrast to the operating parameters, the channel characteristics that exist between a transmitter-receiver pair are hard to control and tend to vary with time.
In order to maintain a given performance level of a wireless link between a transmitter-receiver pair when the channel characteristics vary with time, the operating parameters of the transceiver-receiver pair must adapt to account for the hard to predict variations of the channel characteristics. One technique for maintaining a wireless link involves measuring certain transmission parameters of the wireless link such as delay spread, signal-to-noise ratio, channel-to-interference (C/I) ratio, Doppler shift, and k-factor and then using the measured parameter(s) to predict how the current operating parameters should be changed to maintain the performance level of the wireless link. Predicting how a current operating parameter(s) should be changed often involves consulting look-up tables or analytical performance curves, which try to estimate a future performance level from a given set of operating parameters and/or measured transmission parameters. While this technique works well, the look-up tables and analytical performance curves do not always accurately reflect the performance of an actual wireless link that is effected by a unique set of link conditions and therefore the operating parameter adjustments may not be ideal for the actual link conditions.
Another technique for maintaining a wireless link involves decoding received information and measuring the error rate of the transmission that is being conducted under a given set of operating parameters and channel characteristics. The operating parameters of the transceiver-receiver pair are then adapted in response to the measured error rate. An example of a wireless link that is adapted in response to a measured error rate is disclosed in U.S. Pat. No. 6,072,990 issued to Agrawal et al. (hereinafter Agrawal). In Agrawal, information intended for the receiver is decoded and a word error rate (WER) over a given timeframe is observed (observed WER). The observed WER is compared to upper and lower WER thresholds and the operating parameters of the transceiver-receiver pair are modified if the observed WER fall outside of the WER window that is defined by the upper and lower WER thresholds. Determining how to modify the operating parameters to force the observed WER into the WER window is based on theoretical calculations. Although the techniques disclosed in Agrawal work well, modifications to the operating parameters of the transceiver-receiver pair are made in response to the measured error from a single set of operating parameters. That is, the operating parameters of the transceiver-receiver pair do not change while the error rate information is calculated. Because the performance of the wireless link is measured under a single set of operating parameters, the error rate that will result from modifying the current set of operating parameters is unknown. Therefore, maintaining the link may require multiple changes in the operating parameters before the desired set of operating parameters is found.
In addition to maintaining a wireless link at a minimum performance level, it may also be desirable to improve the performance level of a wireless link between a transmitter and a receiver by, for example, increasing the data rate of the link while still maintaining a minimum error rate. One technique for improving the performance level of a wireless link involves transmitting training frames intended for the receiver at a higher transmission rate than the data frames. The performance of the training frames can then be evaluated under actual conditions before the data frames are transmitted to the receiver at the higher transmission rate. Although this technique gives good performance feedback, the transmission of training frames consumes link bandwidth that may be in limited supply during heavy traffic conditions.
In view of the demand on wireless communications systems to deliver different media types over the same wireless link at reliable performance levels and the large number of variables that effect the quality of each wireless link, what is needed is a system and method for evaluating a wireless link that reflects actual operating conditions and does not waste limited link bandwidth.